As is known, a radio receiver, in particular in a multimedia system of a motor vehicle, is able to receive a radio signal, in particular an AM radio signal, AM being the abbreviation for “amplitude modulation”.
Such an AM radio signal, received in modulated form by a radio receiver, is subjected to various sensors and to matched filtering so that the corresponding demodulated radio signal is able to be played back in good conditions, in particular in the passenger compartment of a motor vehicle.
A person skilled in the art is aware of the operating principle of an AM radio signal, that is to say an amplitude-modulated radio signal, received by a suitable radio receiver, for the purpose of being demodulated and then played back to listeners.
An AM radio signal thus contains a carrier p(t) of the signal, having a frequency fp, satisfying the equation:p(t)=A·sin(2·π·fp)
and a message m(t) to be transmitted satisfying the equation:m(t)=M·cos(2·π·fm·t+φ)
The corresponding modulated signal y(t) then satisfies the equation:y(t)=[1+m(t)]*p(t), that is to say:
      y    ⁡          (      t      )        =            A      ·              sin        ⁡                  (                      2            ·            π            ·                          f              p                        ·            t                    )                      +                            A          ·          M                2            ·              [                              sin            ⁡                          (                                                2                  ·                  π                  ·                                      (                                                                  f                        p                                            +                                              f                        m                                                              )                                                  +                φ                            )                                +                      sin            ⁡                          (                                                2                  ·                  π                  ·                                      (                                                                  f                        p                                            -                                              f                        m                                                              )                                                  +                φ                            )                                      ]            
From a spectral point of view, starting from a message m(t) formed of a plurality of frequencies, the amplitude of the Fourier transform FFT of the modulated signal y(t) containing the message m(t), as a function of frequency. F, is shown in FIG. 1.
To demodulate the AM radio signal, the latter is returned to baseband, such that the message m(t) is centered around 0 Hz, as shown in FIG. 2.
One known problem lies in the fact that the AM radio signal may contain a message m(t) corresponding to voice or corresponding to more complex audio content, such as music.
As is known, an audio signal of “voice” type is scarcely modulated and has a very narrow useful bandwidth. By contrast, an audio signal of “music” type exhibits high modulation and has a wide useful bandwidth.
Now, in practice, amplitude-modulated radio signals often contain audio signals of “voice” type.
One drawback of known amplitude-modulated radio signal filtering techniques lies in the fact that the audio signals of voice type are insufficiently filtered. Specifically, in an AM radio signal transmitting the voice of the speaker during a telephone conversation for example, when the speaker stops talking, the only thing remaining in the AM radio signal is the noise that it contains. This noise is not filtered because the filters applied in the prior art essentially relate to high frequencies.